Push-through furnace with graphite rod heating

ABSTRACT

An electrically heated push-through furnace for the sintering of powder metallurgy molded parts and containing an elongated chamber is provided with a plurality of graphite rods parallel to the longitudinal axis of the chamber. The rods do not touch the walls of the chamber.

United States Patent inventors Appl. No.

Filed Patented Assignee Priority Fritz Krall Hanau/Main;

Fritz Kalbfleisch, Lieblos, both of Germany 854,149

Aug. 29, 1969 Sept. 28, 1971 Deutsche Gold-und Silber-Scheideanstalt vormals Roessler Frankfurt (Main), Germany Sept. 27, 1968 Germany PUSH-THROUGH FURNACE WITH GRAPHITE ROD HEATING 2 Claims, 4 Drawing Figs.

U.S. Cl 13/20,

. 263/28 Int. Cl F27b 9/06, F27b 9/36 [50] Field of Search [56] References Cited UNITED STATES PATENTS 2,149,447 3/1939 Lamm et al 3,429,974 2/1969 Watson et al. 1,528,542 3/1925 Hancock et al... 2,543,708 2/1951 Rice et al Primary Examiner-Bernard A. Gilheany Assistant ExaminerR. N. Envall, .Ir. Attorney-Cushman, Darby & Cushman ABSTRACT: An electrically heated push-through furnace for the sintering of powder metallurgy molded parts and containing an elongated chamber is provided with a plurality of graphite rods parallel to the longitudinal axis of the chamber The rods do not touch the walls olthe chamber.

PATENTEUSEPZBIQYI 8,609,199

sum 10F 2 77//// a 5W 1 2i? INVENTORS 1 4/ r2 2/94. 4. [av r2 12 15 as aw ATTORNEYfi PUSH-THROUGH FURNACE WITH GRAPHITE ROD HEATING Push-through ovens for sintering molded parts produced by powder metallurgy, especially such parts made from hard metals, have been known since the beginning of powder metallurgy.

They consist of a heat insulation jacket-surrounded carbon tube whose ends are fastened in water cooled contact rings and which are heated electrically. The sintered material is put in the so-called graphite boats and pushed through the carbon tube. The disadvantage to this arrangement is that the graphite boats form a shunt to the resistance of the carbon tube serving as the heating body. The magnitude of this shunt is not constant on account of the varying distance of the points of support. Its alteration has as a consequence fluctuations in temperature which disadvantageously affect the uniformity of the sintered product.

As a remedy, furnace constructions are known which use in place of the current carrying carbon tubes elongated heating chambers made of fire-resistant brick on the walls of which are placed metallic heating elements, mostly of molybdenum. Also in these furnaces the sintered material is placed in graphite crucibles because this material is optimal with regard to resistance to heating cost and chemical nature for the heat treatment of the sintered materials arising predominantly from carbides. The abundant existence of graphite in the heating channel leads, however, to the quick carburizing of the heat conductor which on this account has a very reduced life of the latter.

In order to avoid the above-described disadvantages, the art has gone to replacing the originally used carbon tube by a strong-walled tube made of graphite, whose heating current, however, is allowed to flow not in the axial direction but perpendicularly thereto by means of induction coils in the peripheral direction. While this type of furnace fulfills the thermal and material prerequisites, it has the disadvantage that the heat energy must be converted from the network frequency to the middle frequency. The energy loss in the transformation is especially noticed in the customary uninterrupted method of operation of the push-through oven and, apart from the investment cost for a transformer is especially disadvantageous.

It has now been found that the disadvantages of the pictured furnaces can be avoided provided several graphite heating rods are arranged in the oven chambers parallel to their elongation. The installation takes place in such manner that in no place is there contact of the heating rods and boats. This kind of construction combines the advantages of the known furnaces without showing their disadvantages.

The drawings illustrate constructions of push-through ovens produced according to the invention.

FIG. I is a schematic view partially in section of one form of furnace useful with a protective gas atmosphere according to the invention;

FIG. 2 is a fragmentary and partially cutaway sectional view taken along the line 2-2 of FIG. 1;

FIG. 3 is a schematic view partially in section of another form of furnace useful under vacuum conditions and embodying the invention; and

FIG. 4 is a fragmentary schematic view partially cut away and partially in section taken along the line 4-4 of FIG. 3.

Referring more specifically to FIGS. 1 and 2 of the drawings, there is provided a furnace 2 comprising a chamber 10 formed of fire resistant material. The base 19 of the chamber is provided with a longitudinal channel 11. The height of the chamber is so selected that above the graphite boats 12 and parallel to the furnace axis there are provided several (usually two) graphite rods 13, while in the channel 11 below the base of the chamber there is provided at least one heating rod 14 of graphite. The ends of these heating rods are fixed in water cooled copper rings 15 which are suitably linked with the electrical current connections (not shown).

Since the heating rods consist of the same material as the boats, chemical influence is avoided and because on the other hand they do not touch the furnace walls In any place, one IS able to be free in the choice of materials for the building of the chamber. It has proven desirable to make the slides 16 of silicon carbide and to build the remaining masonry 17 of highly porous alumina brick as well as to surround the chamber with the customary heat-storing bricks 18.

This combination is advantageous if the heating chamber is operated with a protective gas.

For many qualities of hard metals the sintering is accomplished under a vacuum. Push through furnaces for sintering of hard metals under a vacuum have not been known up to now.

The apparatus of the invention illustrated by FIGS. 3 and 4 is especially suited for use with vacuum sintering. In a vacuum vacuum-push-through-sintering furnace designated generally at 2A, the walls 20 of the heating chamber 21 are built of graphite sheets which are surrounded with a heat insulation of fibrous graphite 22. The finished lined chamber is slid into a cylindrical container 23 whose covers 24 and 25 are furnished with the current and water connectors 26 for the contact rings 27 of the graphite rods 28. On the rectangular opening of the cover on one side there is provided an inlet channel 29 for the push mechanism (not shown) and on the other side the exit channel 30 for the boats provided with water cooling. Both channels carry on their ends vacuum locks of known type through which the individual boats enter or are withdrawn while the chamber is kept airtight or hermetically sealed. Consequently the tanks, entrance channels and exit channel can remain constantly evacuated by suitable means, as is evident, so that a continuous throughput operation is obtained under vacuum.

What is claimed is:

1. In a push-through furnace for boats containing sinterable powder metallurgical objects provided with electrical heating and comprising an elongated fireproof heating chamber having surrounding heat insulation, the improvement comprising: said fireproof heating chamber being defined by walls consisting essentially of graphite, and a plurality of graphite heating rods arranged in said heating chamber, said graphite rods being disposed parallel to the longitudinal axis of said heating chamber and further being spaced inwardly from and free from contact with the walls whereby said rods are disposed within said heating chamber.

2. A push-through furnace according to claim I wherein the fireproof chamber is hermetically sealed and placed under a vacuum. 

1. In a push-through furnace for boats containing sinterable powder metallurgical objects provided with electrical heating and comprising an elongated fireproof heating chamber having surrounding heat insulation, the improvement comprising: said fireproof heating chamber being defined by walls consisting essentially of graphite, and a plurality of graphite heating rods arranged in said heating chamber, said graphite rods being disposed parallel to the longitudinal axis of said heating chamber and further being spaced inwardly from and free from contact with the walls whereby said rods are disposed within said heating chamber.
 2. A push-through furnace according to claim 1 wherein the fireproof chamber is hermetically sealed and placed under a vacuum. 